Effect of Reducing Allochthonous P Load on Biomass and Alkaline Phosphatase Activity of Phytoplankton in an Urbanized Watershed, Michigan
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Lake and Reservoir Management, 29:116–125, 2013 C Copyright by the North American Lake Management Society 2013 ISSN: 1040-2381 print / 2151-5530 online DOI: 10.1080/10402381.2013.800173 Effect of reducing allochthonous P load on biomass and alkaline phosphatase activity of phytoplankton in an urbanized watershed, Michigan John T. Lehman,∗ Jonathan P. Doubek, and Elliot W. Jackson Department of Ecology and Evolutionary Biology, Natural Science Building, University of Michigan, Ann Arbor, MI 48109-1048 Abstract Lehman JT, Doubek JP, Jackson EW. 2013. Effect of reducing allochthonous P load on biomass and alkaline phosphatase activity of phytoplankton in an urbanized watershed, Michigan. Lake Reserv Manage. 29:116–125. Phosphorus (P) concentrations have declined in the Huron River in southeast Michigan subsequent to first a municipal ordinance, and later state legislation, restricting the application of commercial fertilizers containing phosphate. One management objective of the environmental policy was to reduce eutrophication of surface waters and nuisance blooms of cyanobacteria, particularly in large impoundments along the Huron River. Alternative management theory contended that nuisance algal conditions resulted from internal loading of P within the lakes rather than the allochthonous nutrient load. In summer 2012 during conditions of relative drought, we had an opportunity to test whether reduced river P was intensifying P limitation among phytoplankton in impoundments receiving the river water. We compared chlorophyll concentrations and alkaline phosphatase activity measured in 2012 to measurements at the same sites obtained using identical methods in 2004 and 2005, before P reductions were implemented. A large bloom of Aphanizomenon developed in 2012 after a polymictic episode mixed high concentrations of P from the anoxic hypolimnion throughout the lake. Algal biomass was higher and alkaline phosphatase activity levels in 2012 were significantly lower at all sites than in 2004 and 2005, despite reduced supply rate of riverine P. The results confirm that reducing nonpoint source P runoff in the upstream catchment alone is not an efficacious management strategy for these lakes. Key words: algae, alkaline phosphatase, cyanobacteria, eutrophication, P limitation Seasonal algal blooms in impoundments of the Huron River sediments rather than those from river inputs was the great- Downloaded by [John T. Lehman] at 05:53 07 June 2013 near Ann Arbor, Michigan, have been recognized as an en- est factor inducing the blooms (Lehman 2011). An exper- vironmental problem for many years. In an effort to reduce imental program of hypolimnetic water discharge prior to nonpoint source loading of phosphorus (P) to the river, the development of anoxia proved to effectively eliminate mid- city of Ann Arbor enacted an ordinance in 2007 banning summer cyanobacteria blooms and replaced them with a the use of lawn fertilizer containing P unless soil test data diatom community (Ferris and Lehman 2007, Lehman et al. identified a specific chemical need for the element (Lehman 2009, Goldenberg and Lehman 2012). The results prompted et al. 2009). Similar legislation was later adopted statewide the municipality with jurisdiction over the lake and its out- in Michigan as Public Act 299 of 2010. Statistical analyses let hydroelectric dam to manage the lake by hypolimnetic after 3 years of post-ordinance observation revealed signifi- withdrawal in subsequent summers, and water quality was cant reductions in river P (Lehman et al. 2011). greatly improved from 2008 to 2011. Meanwhile, studies in Lake Ford, the Huron River impound- Scientific interpretations were confounded, however, by the ment historically producing the largest nuisance cyanobac- coincidence that the ordinance restricting lawn fertilizers teria blooms, indicated that internal loading of P from anoxic containing P had been enacted in the upstream drainage in 2007. It remained unresolved whether the P restrictions and reduced allochthonous loading rates had been respon- ∗Corresponding author: [email protected] sible, at least in part, for increased P limitation of lake 116 Alkaline phosphatase algae and reduced nuisance conditions. Bioassay experi- increased cyanobacteria biomass and reduced P limitation ments performed in 2005 demonstrated that P was the prin- by those algae if the causation theory was correct. cipal element limiting algal biomass development in Ford Lake (Ferris and Lehman 2007), and bioassays in 2008 re- vealed that Fragilaria achieved dominance over Aphani- zomenon through superior competition for limiting P (Mc- Materials and methods Donald and Lehman 2013). Field site Drought conditions across the Midwest in 2012 prevented Our field site was the middle portion of the Huron River the use of hypolimnetic discharge as a management tool in catchment in southeastern Michigan (United States Geolog- 2012, and Ford Lake again developed cyanobacteria blooms. ical Survey [USGS] Cataloging Unit 04090005; Fig. 1). We took this opportunity to use an existing archive of water We studied the 2 largest impoundments, Ford Lake and chemistry data, including alkaline phosphatase (AP) activity ◦ ◦ Belleville Lake. Ford Lake (42.21 N, 83.56 W) has a sur- rates, to assess the biochemical evidence for P limitation of face area of 4.039 km2 and mean depth of 4.3 m; Belleville post-ordinance algal communities in the absence of whole ◦ ◦ Lake (42.21 N, 83.49 W) has surface area of 3.91 km2 and lake manipulation. Baseline data for AP activity, chlorophyll mean depth of 5.1 m (Nriagu et al. 2001). Both impound- (Chl-a), and water chemistry measured in 2004–2005 (pre- ments were constructed in the early 20th century for hy- ordinance) were compared with data collected by identical droelectric power production, and they continue to produce methods in 2012. We also compared AP activity in 2012 with power commercially. The impoundments are operated as 2006, which was the first year that experimental trials of hy- run-of-the-river, meaning that stage heights are held nearly polimnetic withdrawal were conducted (Ferris and Lehman constant by controlling water flow through turbines and au- 2007). Because AP production is induced by phosphate lim- tomated hydraulic flood gates. itation, the enzyme is a useful quantitative index of P limi- tation (Healey and Hendzel 1979, Gage and Gorham 1985). Each of the impoundments was sampled at or near its outlet Chrost and Overbeck (1987) found that during summer, AP dam at stations designated F3 and B2 for Ford Lake and activity in the photic zone of a eutrophic lake was mainly Belleville Lake, respectively. Each lake was also sampled produced by phytoplankton rather than bacterioplankton. at an additional site, designated F2 and B1, respectively They also found that AP activity was inversely related to (Fig. 1). The Huron River was sampled at 3 sites where available dissolved inorganic P and positively related to al- runoff was within the jurisdiction of the lawn fertilizer or- gal biomass, measured as adenosine triphosphate (ATP) in dinance, designated A, B, and F, as well as an upstream their study. Other authors have found similar patterns be- site, named CTL, to provide control information about river tween AP activity, inorganic P, and biomass measured as water quality entering the study region (Fig. 1). Chl-a (Labry et al. 2005). With this foundation, our aim was to test 2 apriorihypotheses: Null Hypothesis: AP activity in 2012 was unchanged from Water sampling and analysis baseline levels measured in 2004 and 2005 despite sig- Downloaded by [John T. Lehman] at 05:53 07 June 2013 Water samples were collected weekly at each sampling site nificant reductions in P loading into the lakes from the from May to September 2012. Only immediate subsurface river, signifying no change in the P limitation status of (0 m) samples were used for this study. Archival data existed phytoplankton. at weekly or shorter intervals for each site from prior years. Causation Hypothesis: Lake phytoplankton exhibited sig- Again, only immediate subsurface samples were used for nificantly increased AP activity in 2012, signifying in- comparison. Nutrient chemical analyses were identical to creased P limitation within the algal community subse- those described by Ferris and Lehman (2007), Lehman et al. quent to efforts reducing nonpoint source loading of P (2009), and Lehman (2011) and included soluble molybdate- from the upstream catchment. reactive P (SRP), total dissolved P (DP), total P (TP), soluble − molybdate-reactive Si (SRSi), and nitrate (NO3 ). The SRP Application of the scientific method required that we look was measured as molybdate-reactive phosphate in filtrate for the existence of evidence that explicitly should not exist (10 cm path length at 885 nm, 3 replicates, detection limit if the test theories were correct. In this case, the manage- 0.2 μg/L as P); DP and TP were measured as SRP after ment theory being tested was that successful reduction of first oxidizing filtrate (DP) or unfiltered water (TP) with P levels in the Huron River (reduced external loading) had potassium persulfate at 105 C for 1 h. reduced eutrophic conditions and that it both increased the phosphorus deficiency and reduced the biomass of nuisance Samples collected from lake sites were also measured for algae in Ford Lake. We therefore would not expect to find Chl-a and AP activity. For Chl-a, pigment was extracted 117 Lehman et al. Figure 1.-Site map with river and lake sampling sites identified. by macerating